UTOPIA PLANITIA

northern lava plain on the planet Mars. It was the landing site of the U.S. Viking 2 planetary probe (1976). The boulder-strewn plain of Utopia Planitia, at 47.97 N and 225.74 W, superficially resembles the Viking 1 landing site in Chryse Planitia (q.v.). Soil samples from the two sites are nearly identical in composition, which is probably the result of a mixing of windblown dust from wide regions of the planet. The Utopia plain differs from the Chryse area in that it has a system of shallow troughs, which may be part of a large-scale polygonal patterning possibly associated with ice wedging by permafrost. The vesicular boulders detected at the Utopia site may be either local lavas or rocks ejected from the nearby crater Mie. Photographs transmitted by the Viking 2 lander showed the presence of a thin layer of white ground frost, composed of water, for about 100 days during each of the two Martian winters observed. The frost is probably deposited when carbon dioxide and water condense on airborne dust and precipitate. The carbon dioxide portion is thought to evaporate during exposure to sunlight, leaving only water frost.